This invention relates to analyzing the distortion present at the outputs of a quadrature demodulator.
It is important in the design and alignment of quadrature modulation systems, such as quadrature amplitude modulated (QAM) systems to have a useful measure of distortion. The conventional solution to this problem is to measure bit error rate (BER), a process requiring the accumulation of an error count for a period of minutes, in the case of a low BER. The present invention measures and displays distortion in real time, greatly facilitating adjustment of a quadrature modulated system.
In accordance, with the present invention, there is provided method and apparatus for displaying measures of distortion present at the outputs of a quadrature demodulator. From each of the in-phase and quadrature channels of the demodulator, there are acquired n parallel bits, m of these bits being the data output of the associated channel, and (n-m) bits being error bits for the channel. The n bits for each channel are converted to signals acceptable to a device such as an oscilloscope which can display them with respect to orthogonal axes. The resulting signals associated with the in-phase and quadrature channels are applied to the orthogonal axes of the display device.
The resulting display corresponds to a graph of the m x m quadrature modulation states plotted on Q (quadrature) and I (in-phase) axes. The presence of various forms of distortion manifests itself as additional points displayed in addition to the m x m modulation states. Adjustments which may be made in the system at points prior to the demodulator output can readily be observed to have an effect on distortion by means of this display.
Further in accordance with the invention, there is generated the absolute value of the (n-m) bits for each channel. The absolute value for each channel is filtered and displayed, as by a meter. The resulting displayed values provide useful measures of distortion for the in-phase and quadrature channels. As with the display on orthogonal axes, adjustments in the quadrature modulation system can be seen in their effect on these displayed distortion values.